An accessory power unit (APU) is typically used on vehicles, such as aircraft, to drive the vehicle's hydraulic pumps and electrical generators. Such APU's are typically driven by working fluid bled from the engine of the vehicle during normal operation, or may be operated by a hydrazine powered turbine. Powering an APU from either of these sources incurs certain penalties. Powering the APU with working fluid bled from the vehicle's engine reduces the working fluid available to the engine thereby reducing the engine's efficiency. Relying on hydrazine to power the APU incurs a weight penalty due to the added weight of the tanks of hydrazine. Thus, either method of driving the APU's of the prior art significantly impacts the vehicle's efficiency.
What is needed is an accessory power unit system that minimizes the penalties on engine operating efficiency and weight of a vehicle.